The power factor of an AC electric power system is the ratio of the real power flowing to the load to the apparent power in the circuit and is a dimensionless number between 0 and 1. It is desirable for the power factor to be as close to 1 as possible.
The power factor correction (PFC) circuits are often used within power supply applications in which AC/DC rectification is performed. Such rectifying arrangements typically comprise a full wave voltage rectifier (usually a diode bridge) and an output capacitor to provide regulation of the output waveform at the output bus. This type of rectifying arrangement only draws current from the AC supply when the full wave rectifier voltage is greater than the voltage across the output capacitor. This is unsatisfactory as it gives an inefficient current profile of the input AC current consisting of separated narrow pulses of current having large peak values. The high harmonic content of this current profile gives a low power factor (typically 0.5) of the rectifying arrangement as a whole.
The power factor is improved by applying a PFC circuit between the diode bridge and the output capacitor. Such a PFC circuit essentially comprises an inductor followed by a diode, with a switch (typically an FET) connected between the inductor and the diode to ground.
By rapidly switching the switch on and off, the inductor is repeatedly first connected directly to ground via the switch and then connected to the output capacitor (via the diode) when the switch is turned off. When the switch is on the current flow through the inductor increases and, during the subsequent time period in which the switch is off, the current decreases, effectively pushing current through the diode to charge the output capacitor. By adjusting the on and off times of the switch by a suitable power factor control circuit the output voltage may be adjusted to a fixed, desired value (target value), although the output voltage is always higher than the input voltage because of the action of the diode in conjunction with the “boosting” action of the inductor.
FIG. 1 shows such a known power factor correction circuit 125, based on a boost converter topology. A smoothing capacitor 104 filters a rectified AC input voltage (typically from a bridge rectifier) that is measured by a voltage divider 105, 106. The rectified input voltage is applied to an inductor 101. A secondary winding 102 detects the zero crossings of the current through the inductor 101. A current sensing resistor (shunt) 108 connected to the source of a switch 107 (typically a FET) allows the detection of the inductor peak current to determine a possible over-current condition. In parallel with an output capacitor 111, a second voltage divider 109, 110 is arranged to measure the DC output voltage and a surge condition, for example, due to by load variations.
The above four measurements that take place in the power factor correction circuit 125 by means of four measuring inputs 117, 118, 119 and 120 of an electronic control circuit 116. The control circuit 116 additionally has an output 121, through which the switch 107 is controlled. The electronic control circuit 116 is typically arranged as an ASIC. A total of five pins are used for power factor correction.
In this power factor correction circuit 125 the rectified input voltage is fed to the inductor 101. The inductor 101 is by means of the switch 107 either loaded or unloaded. The on-time of the switch 107 and thus the load time of the inductor 101 is controlled based upon a comparison of the measured DC output voltage Vbus with a fixed reference voltage. The switch 107 is turned off to discharge the inductor 101 until the current through the inductor 101 has fallen to zero (as detected by the secondary winding 102). The switch 107 is cycled with a much higher frequency (at least 10 kHz) than the frequency of the mains voltage (typically 50 Hz) and the frequency of rectified DC input voltage (typically 100 Hz)
To reduce costs power factor correction arrangements which include an electronic control circuit with only a single pin for receiving measurement inputs are know—for example from DE 102004025597 and WO 2011009717. Obtaining an indication of the mains voltage with such a single input pin PFC approach is difficult. Detection of low mains voltage is important, for example, as it may indicate a power failure—see, for example Standard VDE0108.
It is an object of an embodiment of the present invention to provide improved detection of low mains (or other input) voltage to a lamp. It should, however, be appreciated that the invention is applicable to PFC circuits that power devices other than lamps.